Clean up unused members in WebRtcVideoEngine classes

Following the split of the two-way video channel into separate send and
receive channels, several members and methods became unused or redundant.

This CL:
- Removes unused private methods Construct and
  FillBandwidthEstimationStats from WebRtcVideoSendChannel.
- Removes unused member variables last_unsignalled_ssrc_creation_time_ms_
  and rtp_config_ from WebRtcVideoSendChannel.
- Removes unused member variable rtp_config_ from
  WebRtcVideoReceiveChannel.
- Removes redundant initialization of rtcp_mode in
  WebRtcVideoReceiveChannel::ConfigureReceiverRtp.

Bug: None
Change-Id: Ice3aa1788f60a2c192c271e42d503e17fc4c649c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/454702
Auto-Submit: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47098}